Findings from a large, Danish study provide compelling evidence that testing for human papillomavirus (HPV) is more effective for identifying older women with a high risk of developing cervical cancer.

Cervical cancer is caused by HPV infection, which is spread through sexual contact. Two specific types -- HPV 16 and HPV 18 -- are believed to be responsible for up to 70 percent of cases worldwide.

HPV is fairly common among younger women, but in most cases infection is transient and does not pose a health risk.

Infection tends to be rarer and more persistent in older women, however, and infection later in life is increasingly recognized as a major risk factor for cervical cancer.

In the newly reported study, women between the ages of 40 and 50 who tested positive for HPV had a greater than 20 percent chance of developing cervical cancer within 10 years. Most of these older women with positive HPV tests had concurrent Pap smear results that were negative.

“We have documented that a single HPV test can actually predict older women at risk for cervical cancer better than a single Pap smear can,” says researcher Susan Kruger Kjaer, MD.

“Based on these results, we feel that an HPV test would benefit older women, whether or not that test is used in conjunction with Pap smears, or used by itself as an initial screen.”

Kjaer and colleagues from the Danish Cancer Society compared HPV and Pap smear screening in two Danish populations -- 8,656 women between the ages of 22 and 32, and 1,578 women between the ages of 40 and 50.

Cervical samples were collected from all of the women for HPV testing, and they all had multiple Pap smear tests over 10 years of follow-up.

In a pap test, cells from a woman's cervix are sampled and examined under a microscope for abnormalities.

Twenty-one percent of the older HPV-positive women with negative Pap smears developed cervical cancer or precancerous cervical lesions within 10 years, compared with just 1.7 percent of women who tested negative on both screening exams.

As expected, HPV infection was more common in the younger women (17 percent) than in the older women (3 percent). And the older women who tested positive for HPV tended to have more severe cervical abnormalities than the younger women.

The study is published in the Nov. 1 issue of the journal Cancer Research.

The American Cancer Society (ACS) recommends that all women begin having annual Pap test screening within three years of having vaginal intercourse, but no later than 21 years of age.

Beginning at age 30, women who have had three normal Pap tests in a row may choose to be screened every two to three years.

Women over 30 can also opt to have a Pap test and an HPV test. But if both tests are negative, neither test should be repeated for at least three years.

ACS director of breast and gynecologic cancer Debbie Saslow, PhD, tells WebMD that the clinical evidence, including the Danish study, suggests that these guidelines are right on target.

“These are very different tests,” she says. “We know that HPV testing is a better predictor of who will develop cervical disease and Pap tests tell you what is going on today.”

She says Pap testing without HPV remains a very effective cervical cancer screening tool as long as ACS guidelines are followed. This is true for both younger and older women.

HPV testing can tell older women more about their long-term risks of developing cervical disease.

“A negative [HPV] test is almost a sure sign that a woman will have nothing to worry about for at least five years, and maybe longer,” she says. “But if a woman in her mid-30s or 40s has two positive tests over two years, that is a good sign that something is going on, even if her Pap test is normal.”